This application is a national phase of PCT/NL2015/050067, filed Feb. 2, 2015, and claims priority to EP 14153523.7, filed Jan. 31, 2014, and EP 14176734.3, filed Jul. 11, 2014, the entire contents of all of which are hereby incorporated by reference.
The invention relates to an ophthalmic lens to be worn in or on the eye, such as a contact lens or an intraocular lens, the lens having zones with different refractive optical powers.
Presbyopia is a condition in which, with age, the eye is less able to accommodate for focusing on objects at different distances from the eye by bending the natural lens in the eye. Generally, the ability to change the focal distance between objects far away and nearby declines throughout life, from an accommodation of about 20 diopters (ability to change focal distance between infinity to 50 mm away) in a child, to 10 diopters at age 25 (infinity to 100 mm), and levels off at 0.5 to 1 diopter at age 60 (infinity to 1-2 meters only). The ability to accommodate is also lost if the natural lens is removed, a treatment commonly undergone by cataract patients.
The ability to focus on objects nearby and on objects far away without resorting to changing the focal distance of correction optics, e.g. by using reading glasses or multifocal spectacle lenses, can be improved by using multifocal intraocular lenses or contact lenses. Multifocal lenses have different focal distances for near and far vision. In some lenses, the additional optical power of the near vision is provided by a diffractive pattern at the front or back surface of the lens. Diffractive multifocal lenses suffer from dysphotopsia (blur, glare, halos) and light loss. In other lenses, different focal distances are created by providing zones with different refractive power. Use of such lenses entails loss of contrast caused by the transition zone between the zones for near and far vision. The difference in radius of the zones for near vision and far vision (Rnearvision<Rfarvision) results in a step in surface level between the near and far vision zones. In some lenses, it is attempted to design the step to minimize dysphotopsia, for instance by providing a transition zone designed to direct the light which enters the lens through the transition zone to parts in the eye where the light does not disturb vision.
Because in diffractive multifocal lenses the additional optical power is provided by a diffractive structure added to the refractive lens shape, the ratio between amounts of light diffracted and refracted with different focal distances independent of the location of the lens in or on the eye relative to the pupillary area. When a refractive multifocal is not centered relative to (coaxial with) the pupillary area of the eye, the ratio between light passing through a near vision zone and light passing through a far vision tends to change. In other types of multifocal lenses, such as diffractive lenses, an angular difference between the optical axis of the eye and the pupil axis (angle K) can cause patient dissatisfaction due to optical disturbances.
In U.S. Pat. No. 5,512,220 a lens is described in which optical disturbances at junctions between borders between near vision and far vision zones are avoided by having the borders between the near vision and far vision zones in the form of semi-circular paths ending outside the optical region of the lens.